The present invention relates to a gas sensing element and a gas sensor used for controlling the combustion of an internal combustion engine.
Conventionally, a gas sensor is equipped in an exhaust gas system of an automotive vehicle to control an air-fuel ratio of gas mixture introduced into an internal combustion engine.
A gas sensing element, disposed in the gas sensor, usually comprises a solid electrolytic substrate having oxygen ion conductivity, a measured gas side electrode provided on the solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on the solid electrolytic substrate so as to be exposed to a reference gas.
The gas sensing element obtains a sensing value (e.g., limit current value) representing the concentration of oxygen involved in exhaust gas. The sensing value of the gas sensing element reflects the air-fuel ratio in a combustion chamber of an internal combustion engine.
The internal combustion engine is often left in an inoperative condition for a long time (e.g., several hours and several days). It is conventionally known that the gas sensing element produces an extraordinary output at a moment the engine is operated again after such a long interruption.
This kind of extraordinary sensor output continues for several seconds to several tens seconds after a cold starting up of the engine. During this period, the sensor output shifts with a great extent to the rich side (refer to a later-described characteristic curve (c) shown in FIG. 4).
In response to such an abnormal sensor output (i.e., an extraordinary rich signal), an engine control system adjusts an air-fuel ratio of the gas mixture introduced into the combustion chamber to a lean side.
However, the detected extraordinary rich signal does not reflect an actual air-fuel condition in the combustion chamber. Continuously generating a lean signal during a significant period will result in an excessive increase of oxygen in the combustion chamber. The fuel amount reduces contrarily and will stop the engine due to the shortage of fuel.
The occurrence of such an extraordinary sensor output is generally limited to a first startup operation when the engine is driven after a long-term interruption. Such a problem is no longer found in the second or succeeding startup operations.
To solve the above-described problems, an object of the present invention is to provide a gas sensing element and a gas sensor capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
In order to accomplish the above and other related objects, the present invention provides a first gas sensing element comprising a solid electrolytic substrate. A measured gas side electrode is provided on one surface of the solid electrolytic substrate so as to be exposed to a measured gas. A reference gas side electrode is provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber. And, a water-vapor absorbing member is provided in the reference gas chamber.
The first gas sensing element of this invention is characterized in that the water-vapor absorbing member is provided in the reference gas chamber.
The water-vapor absorbing member is any substance capable of trapping water components and is not limited to a specific material. Details of the water-vapor absorbing member will be explained later.
The first gas sensing element of this invention operates in the following manner.
First of all, it is believed that the abnormal sensor output is produced according to the following mechanism.
The gas sensing element, after being left for a long time (several hours or several days), produces an abnormal sensor output. The magnitude of the abnormal sensor output is dependent on the humidity of an atmosphere in which the gas sensing element is left.
The inventor of the present invention has heated a gas sensing element being left for a long term to check the component of a gas leaving out of this gas sensing element and detected a great amount of H2O adhering on the gas sensing element.
Namely, when a gas sensing element is left in an atmosphere including moisture, water molecules adhere or settle on a reference gas side electrode. The largeness of an abnormal sensor output shows the presence of a great amount of water molecules. It is believed that a large surface roughness of the reference gas electrode allows the water molecules to easily adhere or settle on the surface of the reference gas side electrode. Once the water molecules adhere or settle on the electrode surface, another water molecules easily accumulate thereon through hydrogen bridge.
The gas sensing element in such condition is subjected to heat upon starting the operation of the engine.
As shown in FIG. 2, the supply of heat and the catalytic function of a reference gas side electrode 112 cooperatively activate the water molecules adhering on the surface of a solid electrolytic substrate 12 and decompose them into oxygen atoms and hydrogen atoms. Oxygen atoms, when ionized, move toward a measured gas side electrode 111 across the solid electrolytic substrate 12 as an oxygen ion current. The oxygen ion current thus produced is believed to cause an abnormal sensor output.
Once all of the water molecules have decomposed, no abnormal sensor output is produced. Hence, no problem occurs in the second and succeeding startup operations of the engine as long as no water molecules remain on the surface of the reference gas side electrode.
In view of the above, the present invention provides the water-vapor absorbing member in the reference gas chamber to prevent the water molecules from adhering on the surface of the reference gas side electrode. Thus, it becomes possible to obtain an accurate sensor output reflecting an actual oxygen concentration in the measured gas.
As apparent from the foregoing description, the present invention provides an excellent gas sensing element capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
According to a preferable embodiment of the present invention, the water-vapor absorbing member is provided so as to close an inside space of the reference gas chamber.
This arrangement makes it possible to prevent the water vapor from entering into the reference gas chamber.
The water-vapor absorbing member can be provided at an opening end of the reference gas chamber so as to close the opening end entirely as shown in FIG. 1. In general, the opening end of the reference gas chamber is a place where the temperature is not so increased. Thus, a member not strong against heat can be used as the water-vapor absorbing member.
Furthermore, as shown in FIG. 3, it is possible to provide the water-vapor absorbing member at an intermediate position so as to close a middle part of the reference gas chamber. This arrangement is advantageous in that the water-vapor absorbing member is free from damage when the gas sensing element is installed in a gas sensor.
In any case, it is preferable to the water-vapor absorbing member is disposed entirely along the inside wall of the reference gas chamber so as to prevent the water vapor from reaching the reference gas side electrode.
According to the preferable embodiment of the present invention, the water-vapor absorbing member is provided so as to cover the reference gas side electrode provided in the reference gas chamber (refer to FIG. 5).
This arrangement surely prevents the water molecules from reaching the reference gas side electrode.
According to the preferable embodiment of the present invention, the water-vapor absorbing member is porous.
When the water vapor passes through the porous member, the water vapor collides with a wall surface of a labyrinth formed in this porous member. The wall surface absorbs (i.e., traps) the water vapor and accordingly prevents the water molecules from reaching the reference gas side electrode.
According to the preferable embodiment of the present invention, the water-vapor absorbing member is a porous alumina.
Due to excellent heat durability of alumina, it becomes possible to prevent the gas sensing element from deteriorating when exposed to high-temperature exhaust gas.
Especially, the water-vapor absorbing member made of a porous alumina will show excellent durability when the water-vapor absorbing member is disposed closely to a high-temperature portion (e.g., the reference gas side electrode).
Besides porous alumina, activated charcoal and silica gel are substances preferable for the water-vapor absorbing member. Although its heat durability is not so excellent, the activated charcoal is inexpensive and therefore can be used as a water-vapor absorbing member provided in the vicinity of the opening end of the reference gas chamber. Although its absorbing ability is not so good compared with activated charcoal, silica gel is stable in a high-temperature atmosphere and can be used as a water-vapor absorbing member provided closely to a high-temperature portion.
It is also preferable to use a water-vapor absorbing member made of a porous ceramic.
The present invention provides a gas sensor having a gas sensing element comprising a solid electrolytic substrate, a measured gas side electrode provided on one surface of the solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber. The gas sensor of the present invention is characterized by a cylindrical housing for securely holding the gas sensing element, a reference gas side cover provided at a proximal end side of the housing and having an air introducing hole, a measured gas side cover provided at a distal end side of the housing, and a water-vapor shielding portion or a water-vapor absorbing member provided in an air introducing passage extending from the air introducing hole to the reference gas chamber (refer to FIG. 6).
According to the gas sensor of the present invention, the air entering from the air introducing hole is introduced into the reference gas chamber in the gas sensing element. The water-vapor shielding portion or the water-vapor absorbing member, provided in an air introducing passage, prevents the water vapor from entering into the reference gas chamber.
Accordingly, it becomes possible to surely prevent the water molecules from entering into the reference gas chamber even when the gas sensing element is left in an inoperative condition for a long time.
As apparent from the foregoing description, the present invention provides an excellent gas sensor capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
According to the preferable embodiment of the present invention, the water-vapor shielding portion is a cover member capable of selectively opening or closing the air introducing passage.
This arrangement is advantageous in that no processing to the reference gas chamber is required. In other words, the present invention provides a gas sensor which is easy to manufacture.
It is preferable that the cover member opens the air introducing passage upon starting the operation of the engine and closes the air introducing passage upon stopping the operation of the engine. Regarding an opening/closing mechanism for the cover member, it is possible to utilize a motor, a servo mechanism, an other actuator as well as a bimetal and a shape memory alloy.
According to the preferable embodiment of the present invention, the water-vapor absorbing member is porous.
When the water vapor passes through the porous member, the water vapor colliders with a wall surface of a labyrinth formed in this porous member. The wall surface absorbs (i.e., traps) the water vapor and accordingly prevents the water molecules from reaching the reference gas side electrode.
According to the preferable embodiment of the present invention, the reference gas chamber of the gas sensing element has an opening end communicating with an inside space of the reference gas side cover, and the water-vapor shielding portion or the water-vapor absorbing member is provided at the opening end of the reference gas chamber.
With this arrangement, it becomes possible to effectively prevent the water vapor from entering into the reference gas chamber.
The present invention provides a second gas sensing element comprising a solid electrolytic substrate, a measured gas side electrode provided on one surface of the solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber, characterized in that an insulating thin film is provided on a surface of the reference gas side electrode.
According to the second gas sensing element of the present invention, the water molecules adhering or settling on the electrode surface is blocked by the insulating thin film and cannot reach the reference gas side electrode. Due to its kinetic energy, the oxygen residing in the reference gas chamber can penetrate through the insulating thin film. Accordingly, the insulating thin film does not give any adverse influence to the performance of the gas sensing element. The second gas sensing element of the present invention can effectively eliminate an abnormal sensor output caused by an oxygen ion current derived from water molecules.
As apparent from the foregoing description, the present invention provides an excellent gas sensing element capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
According to the preferable embodiment of the present invention, a thickness of the insulating thin film is in a range from 1 nm to 10 nm.
This arrangement surely prevents the water molecules from decomposing on the reference gas side electrode.
If the thickness of the insulating thin film is less than 1 nm, it will be difficult to obtain the effects of the present invention. If the thickness of the insulating thin film exceeds 10 nm, the electrode will obtain inappropriate insulating ability and therefore the sensor performance will go worse.
The present invention provides a third gas sensing element comprising a solid electrolytic substrate, a measured gas side electrode provided on one surface of the solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber, characterized in that a surface roughness of the reference gas side electrode is 3 xcexcm at maximum.
This arrangement substantially smoothens the electrode surface, thereby reducing the water molecules adhering or settling on the electrode surface. Thus, the third gas sensing element of the present invention can effectively eliminate an abnormal sensor output caused by an oxygen ion current derived from water molecules.
If the surface roughness of the reference gas side electrode is larger than 3 xcexcm, the magnitude of an abnormal sensor output will become so large that it cannot be handled as an allowable error. The maximum surface roughness, measurable with a surface roughness meter, is a value defined as a difference between a highest position and a lowest position on a measured surface.
As apparent from the foregoing description, the present invention provides an excellent gas sensing element capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
The present invention provides a first method for manufacturing a gas sensing element comprising a solid electrolytic substrate, a measured gas side electrode provided on one surface of the solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber. The first manufacturing method of the present invention comprises the steps of preparing a first green sheet for forming the solid electrolytic substrate and a second green sheet for forming the reference gas chamber, providing a first print portion on the first green sheet to form the measured gas side electrode and a second print portion to form the reference gas side electrode, applying a pressing force on the first green sheet, laminating the first and second green sheets integrally, pressing the first and second green sheets together to obtain a pressed lamination body, and sintering the pressed lamination body. The first manufacturing method of the present invention is characterized in that the pressing force applied on the first green sheet is in a range from 10 MPa to 70 MPa.
By applying the pressing force of 10 MPa to 70 MPa on the solid electrolytic green sheet, it becomes possible to obtain a reference gas side electrode having a smooth surface. Accordingly, water molecules cannot easily adhere or settle on the reference gas side electrode. Thus, it becomes possible to obtain a gas sensing element capable of effectively eliminating an abnormal sensor output caused by an oxygen ion current derived from water molecules.
If the pressing force is less than 10 MPa, it will be difficult to obtain an effect of this invention. If the pressing force exceeds 70 MPa, a significant damage will be given to the green sheet and a cracking will be produced during the sintering process.
As apparent from the foregoing description, the present invention provides an excellent manufacturing method for a gas sensing element capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
The present invention provides a second method for manufacturing a gas sensing element comprising a solid electrolytic substrate, a measured gas side electrode provided on one surface of the solid electrolytic substrate so as to be exposed to a measured gas, and a reference gas side electrode provided on an opposite surface of the solid electrolytic substrate so as to be exposed to a reference gas stored in a reference gas chamber. The second manufacturing method of the present invention comprises the steps of preparing a first green sheet for forming the solid electrolytic substrate and a second green sheet for forming the reference gas chamber, providing a first print portion on the first green sheet to form the measured gas side electrode and a second print portion to form the reference gas side electrode, laminating the first and second green sheets integrally and pressing the first and second green sheets together to obtain a pressed lamination body, and sintering the pressed lamination body. The second manufacturing method of the present invention is characterized in that the second print portion for forming the reference gas side electrode includes 5-10 wt % ZrO2 grains contained in 100 wt % electrode paste.
Mixing ZrO2 grains with a paste of the reference gas side electrode makes it possible to improve the adherence between the solid electrolytic substrate and the reference gas side electrode because ZrO2 grains can integrate with the solid electrolytic substrate during the sintering operation. However, ZrO2 grains possibly increase the surface roughness of the solid electrolytic substrate. Accordingly, the reference gas side electrode will have a surface reflecting the increased surface roughness of the solid electrolytic substrate.
Hence, in forming the print part of the reference gas side electrode, using the above-described electrode paste makes it possible to effectively reduce the surface roughness of the reference gas side electrode without being adversely influenced by the inclusion of ZrO2 grains. Accordingly, the surface of the reference gas side electrode becomes so smooth that molecules cannot easily adhere or settle on the reference gas side electrode. It becomes possible to obtain a gas sensing element capable of effectively eliminating an abnormal sensor output caused by an oxygen ion current derived from water molecules.
If the percentage of ZrO2 grains is less than 5 wt %, the adherence between the solid electrolytic substrate and the reference gas side electrode will deteriorate and cause them to easily peel off. If the percentage of ZrO2 grains is larger than 10 wt %, it will be difficult to obtain an effect of this invention.
As apparent from the foregoing description, the present invention provides an excellent manufacturing method for a gas sensing element capable of accurately detecting the oxygen concentration as well as the air-fuel ratio even after the engine is left in an inoperative condition for a long time.
The electrode paste can include various binders in addition to the electrode materials.
The application of the present invention is not limited to a one-cell type gas sensing element (i.e., comprising a pair of electrodes formed on opposed surfaces of a solid electrolytic substrate as shown in FIG. 1). Therefore, the present invention can be preferably applied to a two-cell type gas sensing element as shown in a later-described seventh embodiment.